netfilter: ipv6: only provide sk_bound_dev_if for link-local addr

yoshfuji points out that sk_bound_dev_if should only be provided
for link-local addresses.

IPv6 getpeer/sockname also has this test, i.e. we will now
only set sin6_scope_id if the original(!) destination
was a link-local address.

Reported-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>